Fuel tanks of automobiles and motorcycles have a complicated shape, and therefore the material of the fuel tank is required to have excellent processability (deep drawability). Further, since the fuel tank is an important safety component of automobiles and motorcycles, it is important that corrosion products constituting a cause of the clogging of a filter not be produced in the material of the fuel tank and that the material of the fuel tank be a material that has no concern of perforation corrosion and on which stable welding can be performed easily.
As a material having these various characteristics, a Pb—Sn alloy-plated steel sheet has been widely used as a material of automotive fuel tanks (see Patent Literature 1). The Pb—Sn alloy-plated steel sheet has stable chemical properties to gasoline and has plating excellent in lubricity, and is therefore excellent in press moldability and excellent also in the weldability of resistance welding such as spot welding and seam welding. However, these days, a material not using lead is demanded in terms of loads on the environment.
As a material not using Pb and having good corrosion resistance and processability, there is a Zn-based plated steel sheet. Zn has stable sacrificial anticorrosion action on a steel sheet, and therefore a Zn-based plated steel sheet has good corrosion resistance to, as well as gasoline, alcohols and organic acids produced by the degradation of gasoline.
Thus, various technologies have so far been proposed as materials for fuel tanks provided with Zn-based plating. For example, Patent Literatures 2 and 3 disclose subjecting a Zn-based plated steel sheet further to chromate treatment mainly using chromic acid and silica for the purpose of improving corrosion resistance. However, these methods are not preferable because a covering film containing a chromate as a main component contains hexavalent Cr, which has high environmental loads.
Patent Literature 4 discloses a surface-treated plated steel sheet containing neither Pb nor hexavalent Cr and including a covering film of a single-layer coating type; this covering film is a covering film containing a resin as a main component and is expected to exhibit moldability; however, there is a concern that continuous workability may be reduced due to the resistance heat generation of the covering film during welding.
Patent Literature 5 discloses a weldable zinc-based plated steel sheet containing a metal pigment in an organic resin covering film; however, due to being provided with an organic covering film, this steel sheet has a problem that a load is imposed on resistance welding and, although welding is possible, productivity is low, a problem that, although corrosion resistance is good, the cost is high, etc.
After all, in conventional technologies, the current situation is that it is difficult to satisfy continuous workability required during fuel tank production and processability and corrosion resistance required as fuel tank characteristics, and at the same time allow low-cost production.
These days, using a high strength steel sheet is investigated from demands for the weight reduction of fuel tanks. For example, Patent Literature 6 discloses a steel sheet for fuel tanks that is excellent in strength-r-value balance, with a tensile strength of more than or equal to 440 MPa and an r-value of more than or equal to 2.0, is excellent in the corrosion resistance of the inner and outer surfaces of the tank, and is excellent also in spot weldability and press processability.
However, in Patent Literature 6, although there is a description on covering treatment of a surface of the steel sheet, there is no description on a strength of the steel sheet or a surface form favorable for processability, nor on conditions of a plating layer, nor on a surface covering film; hence, the steel sheet for fuel tanks of Patent Literature 6 has a concern that processability may not have been sufficiently obtained.
Patent Literature 7 discloses a surface-treated steel sheet for fuel tanks in which a zinc-based alloy plating layer containing one or more of Ni, Co, Cr, and Fe as an alloy component is formed and a Ni plating layer is formed thereon. Although a Ni plating layer is formed on a Zn-based plating layer to enhance corrosion resistance to fuel, studies on the strength of the steel sheet and studies on a surface form and a surface covering film are not sufficiently conducted; hence, the surface-treated steel sheet for fuel tanks of Patent Literature 7 has a concern that processability may not have been sufficiently obtained.
Patent Literature 8 discloses a surface-treated steel sheet for fuel tanks excellent in press processability in which the running out of lubrication of processing oil that is put on the steel sheet during molding is suppressed by controlling the number of concavities and the total area of the surface of the steel sheet. However, the control of the surface form based on the number of concavities and the total area of the surface of the steel sheet does not provide sufficient retainability of lubrication, and there has been a concern that press processability may not be sufficiently obtained.